1. Field of the Invention
The present invention relates to a method, system and program for forming a gray scale image using an image forming system capable of outputting a color image.
An example of an image forming apparatus is an ink-jet printer having a plurality of color inks for outputting a color image. When an image is formed by using subtractive color mixture as is the case with the ink-jet recording apparatus, three basic colors consisting of cyan (C), magenta (M), and yellow (Y) are generally used. Use of this combination of colors allows representation of not only hues of cyan, magenta and yellow but also various other hues. For example, red (R) can be represented by mixing magenta and yellow. By gradually changing the ratio of amounts of inks mixed together, colors over an entire color space can be substantially represented.
However, in practice, it is very difficult to find basic color materials (C, M, and Y) having colors located at ideal coordinates in the color space. Coordinates in the color space of respective color materials deviate more or less from their ideal coordinates, and the deviation varies depending on the type of recording medium used. Further, when more ink is employed to increase the density of a color, the resultant locus in the color space does not necessarily linearly extend toward high color saturation, and the saturation tends to decrease with increasing amounts of ink in a range higher than a particular value of the density of the color. The deviation of the color of a recorded image from ideal coordinates in the color space can occur in many types of printers including the ink-jet recording apparatus. Hereinafter, such a deviation will be referred to as a “recorded color deviation”.
In a printer in which recorded color deviations can occur, recording data is corrected depending on the characteristics of inks and/or a recording medium such that a recorded color becomes as close to an ideal color as possible. However, of various hues to be represented, black and gray (achromatic colors) are difficult to adjust. In the gray scale, a slight increase/decrease in amount of ink can result in a large change in hue perceptible by human eyes. Although black can be represented by mixing three basic colors, it is difficult to obtain black with a sufficiently high density even if maximum possible amounts of the three color inks are used. In recent ink-jet recording devices, to avoid the difficulty described above, black ink is added to the three basic colors. In such an ink-jet recording apparatus, when a gray scale is represented, only the black ink is used or basic color inks are used together with the black ink. Typically, pigment ink is used for the black color while dye inks are used for the basic colors.
FIG. 1 shows output values of respective color inks used to record a gray scale image using a conventional ink-jet recording apparatus. In FIG. 1, the horizontal axis represents the density level for each color of the image, the density level varying from 0 to 255. The vertical axis represents output signal values (0 to 255) of respective ink colors used to achieve the density. As can be seen from FIG. 1, in a low-density range, three colors C, M, and Y are used to form a gray image. The ratio among the output signal values of the three colors is adjusted to prevent the recorded color deviation. For a range in which the input signal value is higher than 176, a black ink (K) is used. Use of the black ink is started when the input signal value is 176. The output signal value for the black ink increases with the input signal value, up to a value of 128, which corresponds to the highest density. In FIG. 1, it is assumed that a pigment ink is used for the black ink. Note that the output curves of FIG. 1 are examples and the shapes of the curves are not limited to those shown herein. Use of the black ink may be started at a value greater or smaller than 176, and the output value of black at the maximum density (255) may be greater than 128 up to 255. The output signal values for the color inks need not be monotonically increased as shown in the example of FIG. 1. Rather, the output signal values for color inks may be decreased after use of the black ink is started.
Ink jet-recording apparatuses are also used to print photographic images that are comparable to silver halide photographs. However, granularity in such photographic images is perceptible to a user. Granularity refers to a visually perceptible rough texture that appears in an image due to ink dots used to record the image on a recording medium. In essence, images with visible granularity are considered low quality compared to silver halide photographs.
To reduce granularity, many recent ink-jet recording apparatuses use a plurality of inks that are similar in color but different in density.
FIG. 2 shows output values of respective ink colors employed by an ink-jet recording apparatus using a plurality of inks that are similar in color but different in density, wherein output values are plotted in a manner similar to FIG. 1. In this example, in addition to cyan (C), magenta (M), yellow (Y), and black (K), light cyan (LC) and light magenta (LM) with low color material density are used. As shown in FIG. 2, in the low density range, the gray scale image is formed using LC, LM and Y. If the high-density inks are used in this range, dots are formed sparsely and a visible granular texture appears. To avoid such granularity, low-density inks are used. When low-density (light color) inks are used, ink dots formed on a recording medium are not easily perceptible. In FIG. 2, as in FIG. 1, the ratio among the output signal values of the three colors is adjusted to prevent the recorded color deviation.
In the middle density range, the output signal values of LM and LC increase with the input signal value, and reach their maximum values. Densities still higher than these values cannot be obtained by using these inks. In the middle density range, an image recorded on a recording medium is fully filled with many dots, and thus granularity due to dots is not easily perceptible. To obtain higher densities, inks of C, M, and K are gradually added. This allows an increase in density while maintaining low granularity. As output values of C, M, and K are increased, the output values of LC, LM, and Y are decreased. Finally, the output value of K exceeds the output value of any other ink and the output value of K is further increased to obtain a high density for black and thus higher quality.
One method for achieving high image quality comparable to that of silver halide photographs is by forming a greater number of dots per unit area using smaller ink droplets. However, when a gray scale image is recorded using such an ink-jet recording device, the problem associated with the recorded color deviation becomes more critical.
If sizes of fired ink droplets are decreased, slight variations in characteristics of the recording (or print) head among colors can greatly influence the printed image. These variations can occur when the recording head is produced. Note that variations in amounts of fired ink droplets, which can occur over time, can also influence the printed image. In particular, when a gray scale image is recorded, a recorded color deviation, that is, a gray balance deviation that cannot be ignored occurs. The amount and the direction of recorded color deviation are uncontrollable, and the recorded color deviation can cause not only a gradual change in lightness or hue but also an abrupt transition (or “color transition”) in color. Note that a color transition tends to occur in a range in which a dominant ink is switched from a low-density ink to a high-density ink.
Since current ink-jet recording apparatuses should be capable of recording high-quality images not only for color photographic images but also for monochrome photographic images, it is desirable to resolve the color deviation and the color transition problems. To achieve high quality in monochrome photographic images, it is also desirable to achieve good gray tones in the low density range and the middle density range, as will be described in detail below.
Many conventional ink-jet recording apparatuses are designed to achieve high quality for color images. For monochrome images, they are designed to achieve high quality, high contrast, and high density when text is recorded. For the middle density range and the low density range, ink-jet recording apparatuses are designed to obtain a gray tone which varies smoothly with the maximum density without having a recorded color deviation. Low granularity is also maintained.
However, in such ink-jet recording apparatuses, the quality of monochrome photographic images is lower compared to silver halide photographs. In monochrome photographic images, it is desirable to have gray tones in the low density range and the middle density range to achieve high quality. However, the hue in those ranges is not sufficiently taken into consideration in the conventional design, and thus a deviation from ideal hue occurs.
FIG. 11 shows the ideal position for gray in the a*b* plane of the CIE−L*a*b* space and also shows the position for a recorded gray color corresponding to an input value of 128 shown in FIG. 2. The a*b* plane refers to a 2-dimensional coordinate system in which hues are represented. Theoretically, an achromatic color is located at the origin of the a*b* plane. In practice, the position in the a*b* plane for neutral gray varies depending on the type of recording medium used. According to panel tests, coordinates of a neutral gray position are a*=1 and b*=0 for paper designed for dedicated use in recording apparatuses (for example, professional photo paper PR-101 available from Canon Kabushiki Kaisha). In FIG. 11, this position is denoted by M.
The position of gray obtained by conventional image processing for a density level of 128 is denoted by P. The position of P is a*≈0.5 and b*≈−5 in coordinates, and thus the position P is apart by d (color difference)≈5 from M. Even such a great difference does not cause a significant problem in color images having various other colors. However, in monochrome photographic images represented using only gray tones, such a difference is perceived by users as a deviation from ideal gray.